The present invention relates to a ship or floating support for transporting or storing liquid in bulk, and fitted with one or more devices for detecting movements of the liquid free surface within the tank(s) of the bulk storage or transport ship.
More particularly, the invention relates to cryogenic transport ships for transporting either liquefied natural gas (LNG) or liquid methane, or else other gases that are maintained in the liquid state at very low temperature, such as propane, butane, ethylene, or any other gas of density in the liquefied state that is lower than the density of water, and that is transported in very large quantities in the liquid state and substantially at atmospheric pressure.
Liquefied gases that are transported at a pressure close to atmospheric pressure need to be cooled to a lower temperature in order to remain in the liquid state. They are then stored in very large tanks that are either spherical, or cylindrical, preferably presenting a cross-section that is polygonal, and in particular tanks that are substantially in the form of rectangular parallelepipeds, said tanks being very thoroughly insulated thermally in order to limit the evaporation of the gas and in order to maintain the steel of the structure of the ship at an acceptable temperature. As a general rule, such ships travel either when fully loaded (95%-98%), or else with a small residue of gas in the bottoms of the tanks (3%-5%) so as to keep the tanks and the insulation system permanently cold, thereby enabling them to be refilled more quickly, and thus avoiding any need to bring the tank down to a low temperature progressively, i.e. slowly, and thus consuming operating time.
Such ships are extremely difficult to operate because of the dangers associated with the gas and the associated risks of explosion. Thus, all of the technical equipment present on board needs to comply with extremely strict standards since the slightest spark runs the risk of leading to deflagration, and such a spark could be created by an impact between pieces made of metal, merely by a switch, or indeed by radio transmission at a power level exceeding a given threshold. All of those restrictions are the subject of very strict standards and equipment must comply with the conditions laid down in the ATEX standards, i.e. explosive atmosphere standards that are known to the person skilled in the art.
On a voyage, the contents of the tanks behave like liquids with free surfaces, and breaking swell type phenomena, known as “sloshing”, can appear within the tank and can become very violent, in particular when waves break against the vertical walls of the tank, and also in particular when they break in the trihedron formed by the junction between two vertical walls and the ceiling of said tank. Such phenomena are particularly sensitive to the fact that the liquids concerned present viscosities that are very low, less than that of water.
These phenomena run the risk of appearing on methane tanker ships and also on anchored storage ships known as floating production storage and offloading (FPSO) ships, not only when sea conditions are rough, but even when the sea is almost smooth, in the event of the liquefied gas cargo entering into resonance with the excitation that is created by the swell to which the ship is subjected, even if the excitation is of small amplitude. In the event of resonance, sloshing can become very violent, and when waves break against the vertical walls or in the corners, there is a risk of damaging the system for confining the liquefied gas, or of damaging the insulation system that is present immediately behind said confinement system.
Sloshing phenomena can occur even under sea conditions that are relatively calm, but in general they appear only at very particular filling levels, with each combined state of significant amplitude of swell, period, angle of incidence, ballasting of the ship, . . . running the risk of becoming dangerous when a tank is at some particular filling level.
Thus, the problem of the present invention is to predict sloshing type phenomena of swell waves breaking within the tanks of ships for transporting or storing liquefied gas, in particular liquid methane or “LNG”, by detecting the phenomena that occur prior to the appearance of said sloshing. In the description below, the term “LNG” is used to designate methane in the liquid state, i.e. liquefied natural gas, while the gaseous state is referred to as “methane” or as “gaseous methane”.
Revealing the presence of these phenomena that occur before the appearance of such sloshing then enables the captain of the ship to modify the behavior of the ship, where appropriate, e.g. by changing its heading or its speed, so as to attenuate the resonance effects that might lead to sloshing that is damaging to the integrity of the ship. In the same manner, for ships that are fitted with means for statically or dynamically attenuating sloshing, e.g. external fins or active ballast systems, or indeed attenuation means that are incorporated directly in the tanks of said ship, revealing the presence of sloshing-precursor phenomena makes it possible to modify and adjust the settings of said systems finely in order to attenuate or even eliminate the unwanted phenomena.
The inventors have tried various devices for detecting movements of the liquid free surfaces inside storage tanks of ships or floating supports, but the sensitivity of such devices leads to information that is not of any use, in particular when using detector devices based on measuring the free area of the inside walls of a tank containing said liquid free surface, using sonars or ultrasound devices.
The problem of such detection results from the free surface of LNG being difficult to detect because of extremely low temperature conditions, and furthermore, in order to be able to analyze the free surface properly in zones that are critical for deducing the risks of essentially damaging sloshing occurring, it would be necessary to install too great a number of detectors.
According to the present invention, the inventors have implemented devices for detecting the movements of the liquid free surface, which devices are appropriate for those circumstances, and are based in particular on the principle of sensors for sensing vibration of a wall that is in direct or indirect contact with said liquid free surface, i.e. a wall to which the vibration of the walls of the tank is transmitted, detection preferably taking place with the help of vibratory accelerometers that measure variation in acceleration g as a function of time.